The present invention generally relates to visual display devices, and more particularly is concerned with filters for enhancing the image contrast of such devices.
With the ever-expanding frontiers of space and aviation, and with modern aircraft now operating at altitudes which only a few decades ago were thought to be impossible, it is becoming increasingly important to overcome some problems introduced by high altitude flight. Currently, a major concern is the improvement of contrast of visual displays within the cockpit. At high altitudes, the ambient light is quite bright and often greatly reduces the image contrast of avionics equipment displays. The contrast reduction is typically observed when ambient light strikes the faceplate or an interior display generating surface of a visual display device and is then reflected back away from the display toward the viewer.
Typically, several types of displays are used in aircraft cockpits. Cathode ray tubes (CRTs), thin film electroluminescent (TFEL) matrix displays, and liquid crystal displays (LCDs) are used, with the CRT becoming by far the predominant display device. While the discussion herein is largely focused on the application of this invention to CRTs in an aviation environment, it is equally applicable to other types of displays which have a faceplate and an interior display generating surface as well as in non-aviation environments. CRTs, whether used for radar, laboratory work or for home television screens, etc., are made in essentially the same way. The tube is usually made of glass and has a front or viewing face. On the inner side of this face is an interior display generating surface of phosphors which are activated by electrons striking them from the rear of the tube to cause them to glow and give off diffuse light. Depending upon the purpose, they are activated selectively in response to input signals to produce images of many types, such as cloud patterns for weather radar, dots, blips or pips for other radar instruments, and photographic-type pictures as in home television.
Ambient light striking the face of a cathode ray tube produces two problems. The more minor problem is refection off the first surface of the glass. In home television, the majority of this reflection is off the first surface of the safety glass. This problem can be reduced to a fairly satisfactory level by producing a low reflecting or "non-reflecting" coating on the glass. Similar measures also have been commonly used in aircraft.
The second and considerably more serious problem is that of the ambient light rays passing through the glass of the tube and striking the phosphors. In addition to being diffuse emitters of light, the phosphors also act as diffuse reflectors. Consequently, the ambient light rays are reflected diffusely off all the phosphors, whether or not they are being activated by the electron discharge of the tube at the time. Since the ambient light, particularly on a bright day in an airplane at high altitude, may be far greater than the light of the activated phosphors, the reflected ambient light may and frequently does completely "wash out" or obliterate the signal. This results from the fact that the shadows, background, or low lights, are illuminated by the ambient light to such an extent that they cannot be distinguished from the signals, or high lights. The contrast is thereby degraded; the image is confused and in some cases completely lost.
Numerous methods and devices have been proposed to enhance the contrast of display devices in environments having bright ambient light. One typical method and device used is shown in U.S. Pat. No. 4,253,737 to A. J. Thomsen on Mar. 3, 1981. This device essentially is an externally structured and patterned light abosorption filter placed in front of the CRT, between the screen and the viewer.
While this system or variations of it have been used for filtering ambient light, they do have serious drawbacks. One major problem with such filters is the fact that they will absorb equally the light generated by the CRT as well as the ambient light and this tends to decrease the degree of contrast enhancement that can be attained.
Consequently, a need exists for improvement in contrast enhancement filters for visual displays in environments having bright ambient light which will result in increased contrast enhancement.